The present invention relates to a suspension control system designed to limit motion of a vehicle body above a spring so as to improve ride quality.
One such suspension control system is disclosed in Japanese patent application No. 9-367760 filed by the applicant of the present application. In this system, accelerometers, level sensors and other sensors are employed to determine terrain conditions, such as bumpy terrain and rough terrain, and to achieve control over dampening performance in response to the terrain conditions.
Terrain conditions are generally classified into smooth terrain, bumpy terrain and rough terrain as shown in TABLE 1. One type of the bumpy terrain (conveniently, referred to as xe2x80x9cgradual bumpxe2x80x9d) has a wavy section as shown in FIG. 10(a). The other type is directed to independent or intentional steep bumps as shown in FIG. 10(b). Such steep bumps (or depressions) may appear in substantially a periodic manner and may have a height of 10 cm to 20 cm and a length of 30 cm to 4 m.
In TABLE 1, the expression xe2x80x9cmedian between medium and largexe2x80x9d is intended to mean median value between medium and large amplitudes.
In the system disclosed in that application, terrain conditions are determined so as to adjust dampening performance in response to the terrain conditions. However, such adjustment is not satisfactory since no consideration is given to time. Where intentional bumps have a fixed distance or length, it is only necessary to vary the amount of dampening in response to the terrain conditions while the bumps continue. In the disclosed system, it is necessary to determine terrain conditions in each control cycle so as to adjust the dampening performance. This approach is cumbersome and is unable to provide an adequate degree of control over the dampening performance.
Typically, when a vehicle passes over a steep bump, it travels at a low speed since such a steep bump has a height of, for example, 10 cm to 20 cm. At that time, a part of the vehicle body above a spring is subjected to substantial heave movement. When the vehicle travels on bumpy terrain, the disclosed system is designed to set a high gain and constantly develop a large damping force. It is to be understood that a higher gain needs be set (that is, a larger damping force is required) when the vehicle passes over a steep bump rather than a gradual bump. However, such a high gain degrades ride quality when the vehicle passes over a gradual bump.
When the vehicle passes over a steep bump, the suspension is susceptible to full rebound and full bumping. To prevent this, it is desirable to increase the amount of dampening. However, such an increase in the amount of dampening results in poor ride quality when the vehicle travels on smooth terrain.
In view of the foregoing, it is an object of the present invention to provide a suspension control system which can more appropriately adjust the amount of dampening in response to terrain conditions.
It is another object of the present invention to provide a suspension control system which can provide high ride quality when a vehicle passes over a steep bump.
In order to achieve the foregoing objects, the present invention provides a suspension control system comprising a shock absorber adapted to be mounted between a vehicle body and a wheel axle and having a variable damping force, an actuator operatively connected to the shock absorber and rendered operative to adjust the damping force developed by the shock absorber, means for detecting terrain conditions and developing a corresponding terrain condition signal having a frequency, and a controller including means for determining terrain conditions in response to the frequency of the terrain condition signal and adapted to control the actuator in response to the terrain conditions as determined by the means for determining terrain conditions, wherein the controller is operable to increase the damping force by a predetermined amount over a predetermined period of time when the means for determining terrain conditions determines that terrain includes a bump.
According to the present invention, when it is determined that terrain includes a bump, a damping force is increased by a predetermined amount. This ensures proper control over dampening performance. Also, this control is effected within a predetermined time period. This retards deterioration in ride quality which may occur as the damping force is increased and thus, achieves more appropriate control over dampening performance. Further, a reduction in time period during which an increased damping force is applied leads to longer life of the shock absorber and better ride quality.
In one embodiment, a vehicle speed sensor is connected to the controller so as to detect vehicle speed. The controller is operable to vary at least either the applied amount or time of a damping force when the terrain condition determination means determines that terrain includes a bump. This control accommodates a change in vehicle motion in response to vehicle speed. Thus, the system is capable of achieving improved and accurate control over dampening performance in response to vehicle speed.
It is preferable to increase a damping force by an amount greater than the predetermined amount in the event that the terrain condition signal has a low frequency and has an amplitude greater than a predetermined level. This retards full rebound and full bumping when (low frequency) terrain includes steep bumps.
In another embodiment, a vehicle speed sensor is connected to the controller so as to detect vehicle speed. When it is determined that the terrain condition signal has a low frequency and has an amplitude greater than a predetermined level, the controller is operable to vary at least either the applied amount or time of a damping force in response to vehicle speed. This control retards full rebound and full bumping when the vehicle passes over a steep bump and accommodates a change in vehicle motion in response to vehicle speed. Thus, the system is capable of achieving improved and accurate control over dampening performance in response to vehicle speed.